The invention relates to a dual drive arrangement for the drive of a vehicle hydraulic pump, especially a power steering hydraulic pump, and a method for controlling the same.
Modern vehicles are equipped with peripheral components such as a power steering system that can be hydraulically powered.
Power steering systems have been a standard feature in vehicles for many years, in light vehicles and in industrial vehicles.
A power system steering assists the vehicle driver by applying a force to the wheels, through a hydraulic steering actuator. Therefore, a lower effort is required for the driver when turning the steering wheel and the vehicle is therefore easier to manoeuvre.
A power steering system typically comprises a hydraulic circuit carrying a pressurized fluid—such as oil—from a tank to the steering actuator, by means of a pump.
Conventionally, the pump is mechanically driven by the vehicle internal combustion engine, generally gear driven or belt-driven by said engine.
Recent years have seen the emergence of hybrid vehicles which help containing the emissions released by IC engines.
Hybrid vehicles are complex and are of different types. However they all rely on the principle of two or more distinct power sources to move the vehicle. Hybrid vehicles combine an internal combustion engine and one or more electric traction motors.
Thus, a hybrid vehicle can be driven by the internal combustion engine or by the electric motor(s) or by both internal combustion engine and electric motor(s).
In urban driving conditions where emissions should be avoided, it is desirable to drive the vehicle on the electric motor(s) while the internal combustion engine is turned off.
This however causes a problem for the vehicular peripheral components such as the power steering that are normally driven by the internal combustion engine.
Power steering systems can be electrically driven by an auxiliary electric motor. This however has several drawbacks linked to the availability of electric power supply and/or to the reliability of electrical components such as the electric motor or the power supply system.
It therefore appears that, from several standpoints, there is room for improvement in power steering systems.
It is desirable to provide an improved arrangement and an improved method to drive a hydraulic pump especially for a power steering system.
The invention relates, according to an aspect thereof, to a dual drive arrangement for the drive of a hydraulic pump for a vehicle having a gearbox driven by an internal combustion engine said gearbox having an output for mechanically driving driven wheels of the vehicle and said vehicle having an auxiliary electric motor for driving the hydraulic pump. The arrangement further has a three way power split system converting input rotation from the gearbox output and from the auxiliary electric motor into an output rotation driving the hydraulic pump.
The three way power split system may comprise an epicyclic gearing system or a differential.
Thus the invention, according to an aspect thereof, provides an arrangement that ensures a hydraulic pump can be driven by two alternate or combined energy sources. By fitting an epicyclic gearing system between on one side a vehicle gearbox output and an auxiliary electric motor and on the other side a hydraulic pump, the invention, according to an aspect thereof, offers a novel arrangement that ensures that the said hydraulic pump can be driven irrespective of the gearbox output rotational status. In other terms, the auxiliary electric motor can drive the hydraulic pump alone or in combination with the gearbox output so as to ensure that the hydraulic pump can be constantly driven even when the internal combustion engine is turned off and even if the vehicle is at standstill. The arrangement of an aspect of the invention is thus especially useful for hybrid vehicles which can be propelled purely by an electric source.
According to an embodiment of an aspect of the invention, the epicyclic gearing system has a central gear with outward facing teeth driven by the gear box, an outer gear with inward facing teeth driven by the electric power source, a planet carrier holding one or more planet gears meshed with the central gear and with the outer gear driving the hydraulic pump.
Preferably, the auxiliary electric motor drives a worm gear that meshes with outward facing teeth provided on the outer gear. Thus, in case of a failure in the electric power source, outer gear is locked while the central gear can be driven by the gearbox.
The electric motor can be powered by vehicle batteries.
The arrangement can suitably comprise an electronic control unit capable of controlling the electric motor.
The arrangement comprises a speed sensor connected to the electronic control unit that senses the rotational speed of the gearbox output shaft.
Another aspect of the invention concerns a method for controlling the said dual drive arrangement driving a hydraulic pump. The method comprises the steps of:                setting a minimal flow value to the hydraulic pump;        sensing the vehicle speed;        
processing the sensed vehicle speed where the processing comprises comparing the sensed vehicle speed with a reference speed value to thereby determine whether the vehicle operates in a first operational mode where the speed vehicle is too low to drive the hydraulic pump alone up to the minimal flow or in a second operational mode where the speed vehicle is sufficient alone to meet at least the minimal flow value;
controlling the operation of the epicyclic gear so that the auxiliary electric motor drives the hydraulic pump in combination with the gearbox output when the vehicle rims under the first operational mode and the gearbox output drives the hydraulic pump alone when the vehicle runs under the second operational mode.
In a first scenario, the gearbox drives the hydraulic pump alone when the vehicle runs under the second operational mode.
In second scenario, the gearbox and the electric motor controlled in a reverse direction drive in combination the hydraulic pump when the vehicle runs under the second operational mode.